Modify response
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Modify response

Fetch and modify response properties which are immutable by creating a copy first.

Last updated Apr 23, 2026 If you want to get started quickly, click on the button below.

This creates a repository in your GitHub account and deploys the application to Cloudflare Workers.

export default {
    async fetch(request) {
        /**
         * @param {string} headerNameSrc Header to get the new value from
         * @param {string} headerNameDst Header to set based off of value in src
         */
        const headerNameSrc = "foo"; //"Orig-Header"
        const headerNameDst = "Last-Modified";

        /**
         * Response properties are immutable. To change them, construct a new
         * Response and pass modified status or statusText in the ResponseInit
         * object. Response headers can be modified through the headers `set` method.
         */
        const originalResponse = await fetch(request);

        // Change status and statusText, but preserve body and headers
        let response = new Response(originalResponse.body, {
            status: 500,
            statusText: "some message",
            headers: originalResponse.headers,
        });

        // Change response body by adding the foo prop
        const originalBody = await originalResponse.json();
        const body = JSON.stringify({ foo: "bar", ...originalBody });
        response = new Response(body, response);

        // Add a header using set method
        response.headers.set("foo", "bar");

        // Set destination header to the value of the source header
        const src = response.headers.get(headerNameSrc);

        if (src != null) {
            response.headers.set(headerNameDst, src);
            console.log(
                `Response header "${headerNameDst}" was set to "${response.headers.get(
                    headerNameDst,
                )}"`,
            );
        }
        return response;
    },
};

export default {
    async fetch(request): Promise<Response> {
        /**
         * @param {string} headerNameSrc Header to get the new value from
         * @param {string} headerNameDst Header to set based off of value in src
         */
        const headerNameSrc = "foo"; //"Orig-Header"
        const headerNameDst = "Last-Modified";

        /**
         * Response properties are immutable. To change them, construct a new
         * Response and pass modified status or statusText in the ResponseInit
         * object. Response headers can be modified through the headers `set` method.
         */
        const originalResponse = await fetch(request);

        // Change status and statusText, but preserve body and headers
        let response = new Response(originalResponse.body, {
            status: 500,
            statusText: "some message",
            headers: originalResponse.headers,
        });

        // Change response body by adding the foo prop
        const originalBody = await originalResponse.json();
        const body = JSON.stringify({ foo: "bar", ...originalBody });
        response = new Response(body, response);

        // Add a header using set method
        response.headers.set("foo", "bar");

        // Set destination header to the value of the source header
        const src = response.headers.get(headerNameSrc);

        if (src != null) {
            response.headers.set(headerNameDst, src);
            console.log(
                `Response header "${headerNameDst}" was set to "${response.headers.get(
                    headerNameDst,
                )}"`,
            );
        }
        return response;
    },
} satisfies ExportedHandler;

from workers import WorkerEntrypoint, Response, fetch
import json

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        header_name_src = "foo" # Header to get the new value from
        header_name_dst = "Last-Modified" # Header to set based off of value in src

        # Response properties are immutable. To change them, construct a new response
        original_response = await fetch(request)

        # Change status and statusText, but preserve body and headers
        response = Response(original_response.body, status=500, status_text="some message", headers=original_response.headers)

        # Change response body by adding the foo prop
        new_body = await original_response.json()
        new_body["foo"] = "bar"
        response.replace_body(json.dumps(new_body))

        # Add a new header
        response.headers["foo"] = "bar"

        # Set destination header to the value of the source header
        src = response.headers[header_name_src]

        if src is not None:
            response.headers[header_name_dst] = src
            print(f'Response header {header_name_dst} was set to {response.headers[header_name_dst]}')

        return response

import { Hono } from 'hono';

const app = new Hono();

app.get('*', async (c) => {
  /**
   * Header configuration
   */
  const headerNameSrc = "foo"; // Header to get the new value from
  const headerNameDst = "Last-Modified"; // Header to set based off of value in src

  /**
   * Response properties are immutable. With Hono, we can modify the response
   * by creating custom response objects.
   */
  const originalResponse = await fetch(c.req.raw);

  // Get the JSON body from the original response
  const originalBody = await originalResponse.json();

  // Modify the body by adding a new property
  const modifiedBody = {
    foo: "bar",
    ...originalBody
  };

  // Create a new custom response with modified status, headers, and body
  const response = new Response(JSON.stringify(modifiedBody), {
    status: 500,
    statusText: "some message",
    headers: originalResponse.headers,
  });

  // Add a header using set method
  response.headers.set("foo", "bar");

  // Set destination header to the value of the source header
  const src = response.headers.get(headerNameSrc);
  if (src != null) {
    response.headers.set(headerNameDst, src);
    console.log(
      `Response header "${headerNameDst}" was set to "${response.headers.get(headerNameDst)}"`
    );
  }

  return response;
});

export default app;

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Modify response

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